Power tongs have been used for decades to make up and break apart oilfield tubular connections. While such power tongs have a variety of configurations and different mechanisms are used to both grip and rotate an upper tubular relative to a lower tubular, such power tongs generally may be classified as being either the closed throat type or the open throat type. Closed throat power tongs provide a tong body which fully encircles the tubular string, so that repeated oilfield threaded connections pass axially through an opening in the closed throat power tong. The body of an open throat power tong, on the other hand, will typically encircle the majority of the oilfield tubular connection, but an open throat is provided in the tong body and also in the rotary ring which rotates within the tong body to allow the tong to be laterally moved on and off the tubular string. Most open throat power tongs are provided with a door which accordingly is opened to expose the open throat of the power tong when the tong is not being operated. The door of a power tong is closed when the power tong is operated to prevent a tong operator from inadvertently engaging the rotating ring, and also to increase the reliable torque output of the power tong by preventing "spreading" of the open throat. In many oilfield operations, open throat power tongs which provide a door pivotally connected to the tong body are highly preferred over closed throat power tongs.
Various types of latching mechanisms have been used in the power tong industry to retain the pivotal door in the closed position. The commonly used latching mechanism in an open throat power tong employs a heavy duty hammer latch mechanism which includes a latch arm pivotally connected to one of the pair of doors positioned on opposing sides of the open throat. Alternatively, a single door may extend across the open throat, so that the latch arm on the door engages a lug on the tong body. In either case, a latch head at the end of the latch arm engages a latch lug or stop to retain the door or the pair of doors in the closed position. The latch head and the latch lug typically have planar surfaces which engage when the door or the pair of doors are in the closed position. The heavy duty latch mechanism and door are sufficient to withstand a substantial lateral force, and thus minimize spreading of the open throat of the power tong. To open the doors, the operator manually grasps a handle secured to the latch arm and pulls the latch arm away from the latch stop to disengage the mating surfaces. With the door or doors opened, a power tong may then be moved laterally on and off a tubular string.
When the open throat power tong is positioned about the tubular string and prior to activating the partial ring, the door with the latch stop is first closed, then the door with the latch arm is manually closed. The latch arm conventionally includes a spring member which biases the latch arm to the closed position relative to its supporting door. By applying a considerable closing force to the door supporting the latch arm, a cam surface on a latch head engages a corresponding cam surface on the latch stop which causes the latch arm to pivot toward an opened position while the latch head moves radially outward from the latch stop. Once the latch arm is pivotally moved to the opened position, the latch head moves radially inward relative to the latch stop so that the planar surfaces on the latch head and the latch stop engage. The spring on the latch arm serves to provide additional force which retains the doors closed.
A significant disadvantage of the power tong door mechanism discussed above is that it requires a large amount of closing force to shut the doors while the latch head moves radially outward with respect to the latch stop, so that the latch head will then be properly positioned so that it may move back radially inward relative to the latch stop and secure the doors closed. This large closing force requires that the door mechanism components be sized both for withstanding the spreading force discussed above, and also to ensure that components are sufficiently rugged to withstand the repeated substantially jarring force which these components endure during closing of the door. A related drawback of this prior art system is that a great deal of effort is required by a tong operator to close the door, which unfortunately increases the tendency for the operator to merely position the door in the partially closed position and not fully latch the door closed. Failure to latch the door closed creates a safety risk, as discussed above, and may also result in tong spreading when high torque is used to make up or break apart the threaded connection. Moreover, it is often difficult for the tong operator to apply the necessary force to close the door under situations where the tong is not at a comfortable working level of, for example, four feet above the rig floor. In some cases, the tong may be positioned six feet above the rig floor and, in those situations, it is difficult for the tong operator to apply a sufficient force to reliably close the tong door.
Another significant drawback of prior art power tong door latch mechanisms is the complexity of the latch mechanism. Various door latch mechanisms have numerous moving parts, including for example biasing springs and other members that may fail. The failure of one of these parts can adversely affect the safety of the door latch mechanism, and a simple yet reliable door latch mechanisms for power tongs has long been desired. One simplistic prior art door latch mechanism included an upwardly projecting stud and a downwardly projecting stud each welded to the top plate and bottom plate of the tong. Each large diameter short stud thus had an axis which ideally was parallel to the axis of the rotary gear. The door included a hook having a generally U-shaped slot therein, with the slot being sized to fit between the diameter of each stud when the door was closed. While simplistic, this design did not significantly reduce spreading since there was essentially line contact or very small area planar contact between the side of the cylindrical stud and the engaging side wall of the hook.
The disadvantages of the prior art are overcome by the present invention. An improved open throat power tong and a door mechanism for such a power tong is hereinafter disclosed. The door mechanism of the present invention significantly reduces or eliminates the amount of force required by the operator to reliably latch the door in the closed position, and the door latch mechanism is efficient and simplistic in operation and use.